For a number of years already there has been an increased desire for matt surfaces in the automotive finishing segment for example. The matt surfaces produce outstanding optical effects in the case, for example, of automobile finishes in matt silver tones, or where matt and glossy auto parts are combined. As described in the BASF Coatings GmbH News Release “BASF Coatings uses mat clearcoats in small series” of Aug. 24, 2010, matt surfaces are generally achieved through addition of matting agents to the clearcoats.
For the matting of coating compositions, use may be made, alongside organic matting agents, based for example on rubber and/or waxes, of various inorganic matting agents, more particularly based on silicates, based on fumed silica, based on precipitated silica or based on silica gel. The matting agents may optionally be surface-modified as well, with modification taking place using various organic components, such as, for example, hard, microcrystalline waxes, plastifying microcrystalline waxes and/or polyethylene waxes, and/or inorganic components, such as polysiloxanes, for example. The different matting agents and the matting agents modified with different components each have different properties and different pros and cons.
The processing of matted clearcoats, however, is a very demanding operation. Many matting agents tend to settle and to form a sediment which is difficult if not impossible to reagitate thereafter. In addition, the matted clearcoats often display inadequate resistance in the face of shearing loads, of the kind, for example, that occur in circuit lines when the paints are used in automotive OEM finishing. This inadequate resistance to shearing loads is then manifested in an unwanted increase in the glossiness of the resultant coatings. Matted clearcoats have therefore to date been used only in relatively small circuit lines for individual series. On account of the growing demand anticipated for matt surfaces, however, the desire exists to improve the matted clearcoats such that they can also be employed in standard OEM finishing.
From the article by M. L. Maule, J. M. Bogdanor in Surface Coatings Australia, 1994, Vol. 31, pages 6-10, it is now known that the sedimentation stability, for example, may be improved through surface treatment of the matting agents. Also described therein is the fact that, generally speaking, the higher the particle size of the matting agent used, the greater the matting effect. An increasing particle size on the part of the matting agents, however, is also accompanied by increased roughness of the surface and by an increased tendency toward bittiness.
EP-B-541 359 describes silica based matting agents and their use in coating compositions for producing coatings having a matt surface. According to that specification it is essential that these silica based matting agents are surface-treated with a mixture of a hard microcrystalline wax, a plasticizing microcrystalline wax, and a synthetic polyethylene wax, so that the matting agents—even after a high shearing load, as for example when incorporating the matting agents into the coating composition—exhibit a significantly improved sedimentation behavior in the coating composition. Details of how the increasing glossiness of the resultant coatings that is associated with a shearing load, particularly in circuit lines when the paints are used in automotive OEM finishing, can be avoided are absent here, as are details of how the operational window, as it is known, may be improved—that is, of how the glossiness of the resultant coating can be made to exhibit as little fluctuation as possible in response to changes in the conditions under which the coating compositions are applied and cured.
The only coating composition exemplified in EP-B-541 359 is a melamine-alkyd resin paint, while other coating compositions, based on polyurethane, for example, are not described, and nor is the combination of the silica based matting agent with rheological assistants based on hydrophilic or hydrophobic silica.
WO97/08250 likewise describes matting agents based on wax-coated silica, for which it is essential to the invention that the pore volume is at least 1.5 cm3/g, the wax content is between 6% and 15%, based on the matting agent, and the wax has a melting point of 85° C. at most. These matting agents are used exclusively in UV curing coating compositions, where they have the advantage that they exhibit a good matting effect and also, at the same time, neither adversely affect the rheology of the coating compositions nor detract from the optical properties of the resultant coating.
It is known, furthermore, from EP-B-922 671 that precipitated silica based matting agents ought to have a particular particle size distribution in order first to achieve a good matting effect but also, second, not adversely to affect the appearance of the resultant coating. In that specification, however, silica gel based matting agents are not used, and the surface modification of the matting agents is not described. Nor is there any description of the combination of the matting agents with rheological assistants based on hydrophobic silica. It is known, however, that unmodified silicas have a tendency toward severe sedimentation in paints, and can therefore no longer be reagitated thereafter at all, or can be reagitated thereafter only with difficulty, and that their stability in the face of shearing loads is inadequate as well.
As already stated, it is indeed known that matting agents based on precipitated silica and coated with wax, more particularly with polyethylene wax, exhibit a substantially improved sedimentation behavior as compared with the corresponding matting agents that have not been surface-modified. However, these wax-modified matting agents frequently cause clouding of the paint. According to EP-B-1 477 457, this is avoided by subjecting the precipitated silica based matting agents to hydrophobic surface modification not with wax but instead with a modified or unmodified polysiloxane. These special matting agents are used in clearcoats, although the only clearcoat exemplified is one based on a urea-formaldehyde resin. Other clearcoats, such as polyurethane paints based on hydroxyl-containing binders and isocyanate crosslinkers, for example, are not described therein. Lastly, there is once again a lack of details as to how it is possible to avoid the increase in the glossiness of the resultant coatings that is associated with a shearing load, and of details relating to improving the operational window.
EP-B-1 171 531 discloses radiation-curable coating compositions for producing matt coatings, comprising as their matting agents wax-modified silica gels in which the wax content is between 15% and 30%, based on the overall matting agent composition, the silica gel has a pore volume of 0.8 to 1.4 cm3/g, and the average particle size is between 2 and 12 μm. These matting agents are very efficient for radiation-curable coating compositions and do not adversely affect the rheology of the radiation-curable coating compositions. The combination of these special matting agents with hydrophobic silica based rheological assistants, however, is not described therein, and nor is the use of the matting agents in thermally curing coating compositions.
EP-B-1 591 492, lastly, describes two-component coating compositions comprising a polyol component, a polyisocyanate crosslinker, and at least one matting agent. Matting agents which can be used in the coating compositions are all known, surface-treated or untreated, organic or inorganic matting agents. The coating compositions are used for producing multicoat finishes having a very good, specific effect, such as a matt metallic, chromelike appearance, for example. Key to producing the multicoat finishes according to EP-B-1 591 492, therefore, is the use of two different basecoats, the first basecoat being a basecoat of the kind used customarily, and the second basecoat comprising special PVD (physical vapor deposition) aluminum pigments in order to obtain a metallic effect.
The problem on which the present invention is based was that of providing coating compositions which exhibit good circuit line stability, i.e., which suffer no increase in the glossiness of the cured coating as a consequence of the shearing load on the coating compositions that occurs in standard automotive OEM finishing when the coating compositions are applied on the line.
This shearing load is tested by pumping the coating compositions around a circuit line while exposing them to a shearing load, the conditions being as follows:
Pressure at the return check valve: 10 bar
Reciprocating strokes per minute: 18
Volume (reciprocating stroke): 0.6 l
Prior to the shearing load in the circuit line (TO=0) and after turnovers (TO) 500, 1250, 1500, and 2000 in the circuit line, the 60° gloss of the coatings is measured. The coating compositions possess sufficient circuit line stability when the gloss at an angle of 60° is increased by not more than 10 gloss units after shearing by pumped circulation in the circuit line.
Moreover, coating compositions are to be provided which exhibit good sedimentation stability, which means that, even after prolonged storage of the coating compositions for three months at ambient temperature, there is no formation of a sediment which is difficult to reagitate, or can no longer be reagitated, thereafter within not more than one hour using a customary laboratory mixer (more particularly Vollrath 370W, model “EWTHV 0.5” from Paul Vollrath GmbH & CO. KG, Hürth, speed 800 revolutions per minute, Lenart disk d=90 mm) with a speed of 800 revolutions per minute.
Furthermore, the coating compositions ought to be able to be processed with what is called an expanded operational window. This means that fluctuations in gloss or specky surfaces as a function of the application conditions of the OEM multicoat system ought to be avoided. More particularly, the gloss of the resultant coating ought to be approximately the same on pneumatic application and on ESTA application. Furthermore, the drying conditions of the basecoat and also the flash-off time of the clearcoat film ought to have little influence—if any at all—on the gloss of the multicoat finish.
Furthermore, the coatings and finishes, especially the clearcoat finishes, ought to exhibit a good appearance, depending on the intended application. This is a key requirement for the use of the coatings and finishes, more particularly the clearcoat finishes, in the particularly technologically and esthetically demanding segment of automotive OEM finishing.
In addition, the new coating compositions ought to be easy to produce with very good reproducibility, and ought not to cause any environmental problems during paint application.